It is possible to set up scale modes indirectly right now by creating progressions such as this Lydian one: ma_IV V I IV
The voice leading can also be set up so that the different cadences sound right, in this case maybe the melody can move from median to subdominant over the I IV progression.
The problem with this approach is that the scale base note doesn't match the real one since it is always tracking the I chord root. Another problem is pedal figuration lines that are often set up to follow the dominant or tonic. Both of those problems should be possible to solve by just a single option for progressions to treat the IV root as tonic (in the Lydian example). I will certainly consider this as a future feature.
It would be really interesting to try to generate such progressions in templates as well. My knowledge of how to use the different modes (in terms of cadences, voice leading etc.) are currently very limited though.
Thanks for the feature request, I am glad that you enjoy the program!